566 
MODERN TRANSPORT OF DETRITUS. 
Dwina of the East also offers examples of such action of transport from south to 
north, whilst the Volga, particularly in its course from Mologa to Yaroslaf, per- 
forms the same operation in another direction, when flowing from north-east to 
south-west. These river beds also offer an analogy, assisting us to explain why 
the large boulders of the northern drift are usually found associated with mud or 
clay. In many places we observed them to be accumulated in groups, sticking in 
the mud, just at the high-water mark, and where they would naturally be left at 
the breaking up of the frost, when the swollen streams flowing at high levels, the 
sharp edges of the ice would become fastened into the muddy banks until they 
dissolved ; whilst the same masses would shoal away from slopes of incoherent 
sand. The granitic boulders so found in fluvial detritus, and often high on the 
sides of the banks, are frequently more rounded and worn than those mementos of 
the ancient northern drift which lie simply on the surface of the lands, a fact 
well-explained by the river blocks having undergone subsequent rolling on the 
river-sides. 
“ Elevated Fluviatile Ridges of Angular- Blocks ”— Another effect of fluvio-glacial 
action must now be explained. Towards the mouth of the Dwina, and about 110 
versts above Archangel, the white carboniferous limestone before described occu- 
pies the banks in horizontal layers, the edges of which are partially covered with 
mud and sand. The limestone is best seen when the water is low, as at the period 
of our visit. About thirty feet above the summer level of the stream, the terrace 
on the river-side is covered for two or three versts by a band of irregularly piled, 
loose and large angular blocks of the same limestone, arranged in a long uniform 
ledge, the surface of which slopes both to the river and to the roadway, so that the 
view of the stream is shut out from the traveller by this ledge. In other words, 
these materials (all purely local) constitute a broken ridge of stones between the 
road and high-water mark. A woodcut will best explain these appearances, 
showing (a) the ancient hillocks of sand above the road- terrace, which is partially 
covered with water at high inundations, (5) the ridge of broken limestone, (c) the 
